A sensor is a device that produces an output signal for the purpose of detecting a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends the information to other electronics, frequently a computer processor.
Sensors are used in everyday objects such as touch-sensitive elevator buttons (
tactile sensor) and lamps which dim or brighten by touching the base, and in innumerable applications of which most people are never aware. With advances in
micromachinery and easy-to-use
microcontroller platforms, the uses of sensors have expanded beyond the traditional fields of temperature, pressure and flow measurement,[1] for example into
MARG sensors.
Analog sensors such as
potentiometers and
force-sensing resistors are still widely used. Their applications include manufacturing and machinery, airplanes and aerospace, cars, medicine, robotics and many other aspects of our day-to-day life. There is a wide range of other sensors that measure chemical and physical properties of materials, including optical sensors for refractive index measurement, vibrational sensors for fluid viscosity measurement, and electro-chemical sensors for monitoring pH of fluids.
A sensor's sensitivity indicates how much its output changes when the input quantity it measures changes. For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1 °C, its sensitivity is 1 cm/°C (it is basically the slope dy/dx assuming a linear characteristic). Some sensors can also affect what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats the thermometer. Sensors are usually designed to have a small effect on what is measured; making the sensor smaller often improves this and may introduce other advantages.[2]
Technological progress allows more and more sensors to be manufactured on a
microscopic scale as microsensors using
MEMS technology. In most cases, a microsensor reaches a significantly faster measurement time and higher sensitivity compared with
macroscopic approaches.[2][3] Due to the increasing demand for rapid, affordable and reliable information in today's world, disposable sensors—low-cost and easy‐to‐use devices for short‐term monitoring or single‐shot measurements—have recently gained growing importance. Using this class of sensors, critical analytical information can be obtained by anyone, anywhere and at any time, without the need for recalibration and worrying about contamination.[4]
it is insensitive to any other property likely to be encountered in its application, and
it does not influence the measured property.
Most sensors have a
lineartransfer function. The
sensitivity is then defined as the ratio between the output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is constant with the units [V/K]. The sensitivity is the slope of the transfer function. Converting the sensor's electrical output (for example V) to the measured units (for example K) requires dividing the electrical output by the slope (or multiplying by its reciprocal). In addition, an offset is frequently added or subtracted. For example, −40 must be added to the output if 0 V output corresponds to −40 C input.
For an analog sensor signal to be processed or used in digital equipment, it needs to be converted to a digital signal, using an
analog-to-digital converter.
Sensor deviations
Since sensors cannot replicate an ideal
transfer function, several types of deviations can occur which limit sensor
accuracy:
Since the range of the output signal is always limited, the output signal will eventually reach a minimum or maximum when the measured property exceeds the limits. The
full scale range defines the maximum and minimum values of the measured property. [citation needed]
The
sensitivity may in practice differ from the value specified. This is called a sensitivity error. This is an error in the slope of a linear transfer function.
If the output signal differs from the correct value by a constant, the sensor has an offset error or
bias. This is an error in the
y-intercept of a linear transfer function.
Nonlinearity is deviation of a sensor's transfer function from a straight line transfer function. Usually, this is defined by the amount the output differs from ideal behavior over the full range of the sensor, often noted as a percentage of the full range.
Deviation caused by rapid changes of the measured property over time is a
dynamic error. Often, this behavior is described with a
bode plot showing sensitivity error and phase shift as a function of the frequency of a periodic input signal.
If the output signal slowly changes independent of the measured property, this is defined as
drift. Long term drift over months or years is caused by physical changes in the sensor.
Noise is a random deviation of the signal that varies in time.
A
hysteresis error causes the output value to vary depending on the previous input values. If a sensor's output is different depending on whether a specific input value was reached by increasing vs. decreasing the input, then the sensor has a hysteresis error.
If the sensor has a digital output, the output is essentially an approximation of the measured property. This error is also called
quantization error.
If the signal is monitored digitally, the
sampling frequency can cause a dynamic error, or if the input variable or added noise changes periodically at a frequency near a multiple of the sampling rate,
aliasing errors may occur.
The sensor may to some extent be sensitive to properties other than the property being measured. For example, most sensors are influenced by the temperature of their environment.
All these deviations can be classified as
systematic errors or
random errors. Systematic errors can sometimes be compensated for by means of some kind of
calibration strategy. Noise is a random error that can be reduced by
signal processing, such as filtering, usually at the expense of the dynamic behavior of the sensor.
Resolution
The sensor resolution or measurement resolution is the smallest change that can be detected in the quantity that is being measured. The resolution of a sensor with a digital output is usually the
numerical resolution of the digital output. The resolution is related to the
precision with which the measurement is made, but they are not the same thing. A sensor's accuracy may be considerably worse than its resolution.
The sensor may to some extent be sensitive to properties other than the property being measured. For example, most sensors are influenced by the temperature of their environment.
Chemical sensor
A chemical sensor is a self-contained analytical device that can provide information about the chemical composition of its environment, that is, a
liquid or a
gas phase.[5][6] The information is provided in the form of a measurable physical signal that is correlated with the
concentration of a certain chemical species (termed as
analyte). Two main steps are involved in the functioning of a chemical sensor, namely, recognition and
transduction. In the recognition step, analyte molecules interact selectively with
receptor molecules or sites included in the structure of the recognition element of the sensor. Consequently, a characteristic physical parameter varies and this variation is reported by means of an integrated
transducer that generates the output signal.
A chemical sensor based on recognition material of biological nature is a
biosensor. However, as synthetic
biomimetic materials are going to substitute to some extent recognition biomaterials, a sharp distinction between a biosensor and a standard chemical sensor is superfluous. Typical biomimetic materials used in sensor development are
molecularly imprinted polymers and
aptamers.[7]
A
chemical sensor array is a sensor architecture with multiple sensor components that create a pattern for analyte detection from the additive responses of individual sensor components. There exist several types of chemical sensor arrays including electronic, optical, acoustic wave, and potentiometric devices. These chemical sensor arrays can employ multiple sensor types that are cross-reactive or tuned to sense specific analytes.[8][9][10][11]
In
biomedicine and
biotechnology, sensors which detect
analytes thanks to a biological component, such as cells, protein, nucleic acid or
biomimetic polymers, are called
biosensors.
Whereas a non-biological sensor, even organic (carbon chemistry), for biological analytes is referred to as sensor or
nanosensor. This terminology applies for both
in-vitro and in vivo applications.
The encapsulation of the biological component in biosensors, presents a slightly different problem that ordinary sensors; this can either be done by means of a
semipermeable barrier, such as a
dialysis membrane or a
hydrogel, or a 3D polymer matrix, which either physically constrains the sensing
macromolecule or chemically constrains the macromolecule by bounding it to the scaffold.
Neuromorphic sensors
Neuromorphic sensors are sensors that physically mimic structures and functions of biological neural entities.[12] One example of this is the
event camera.
MOS technology is the basis for modern
image sensors, including the
charge-coupled device (CCD) and the
CMOSactive-pixel sensor (CMOS sensor), used in
digital imaging and
digital cameras.[17]Willard Boyle and
George E. Smith developed the CCD in 1969. While researching the MOS process, they realized that an electric charge was the analogy of the magnetic bubble and that it could be stored on a tiny MOS capacitor. As it was fairly straightforward to fabricate a series of MOS capacitors in a row, they connected a suitable voltage to them so that the charge could be stepped along from one to the next.[17] The CCD is a semiconductor circuit that was later used in the first
digital video cameras for
television broadcasting.[18]
The MOS
active-pixel sensor (APS) was developed by Tsutomu Nakamura at
Olympus in 1985.[19] The CMOS active-pixel sensor was later developed by
Eric Fossum and his team in the early 1990s.[20]
MOS image sensors are widely used in
optical mouse technology. The first optical mouse, invented by
Richard F. Lyon at
Xerox in 1980, used a
5μmNMOS sensor chip.[21][22] Since the first commercial optical mouse, the
IntelliMouse introduced in 1999, most optical mouse devices use CMOS sensors.[23]
^Bennett, S. (1993). A History of Control Engineering 1930–1955. London: Peter Peregrinus Ltd. on behalf of the Institution of Electrical Engineers.
ISBN978-0-86341-280-6The source states "controls" rather than "sensors", so its applicability is assumed. Many units are derived from the basic measurements to which it refers, such as a liquid's level measured by a differential pressure sensor.{{
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^Bǎnicǎ, Florinel-Gabriel (2012). Chemical Sensors and Biosensors:Fundamentals and Applications. Chichester, UK: John Wiley & Sons. p. 576.
ISBN978-1-118-35423-0.
^Svigelj, Rossella; Dossi, Nicolo; Pizzolato, Stefania; Toniolo, Rosanna; Miranda-Castro, Rebeca; de-los-Santos-Álvarez, Noemí; Lobo-Castañón, María Jesús (1 October 2020). "Truncated aptamers as selective receptors in a gluten sensor supporting direct measurement in a deep eutectic solvent". Biosensors and Bioelectronics. 165: 112339.
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^Boyle, William S; Smith, George E. (1970). "Charge Coupled Semiconductor Devices". Bell Syst. Tech. J. 49 (4): 587–593.
doi:
10.1002/j.1538-7305.1970.tb01790.x.
^Eric R. Fossum (1993), "Active Pixel Sensors: Are CCD's Dinosaurs?" Proc. SPIE Vol. 1900, p. 2–14, Charge-Coupled Devices and Solid State Optical Sensors III, Morley M. Blouke; Ed.
M. Kretschmar and S. Welsby (2005), Capacitive and Inductive Displacement Sensors, in Sensor Technology Handbook, J. Wilson editor, Newnes: Burlington, MA.
C. A. Grimes, E. C. Dickey, and M. V. Pishko (2006), Encyclopedia of Sensors (10-Volume Set), American Scientific Publishers.
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